A form of spectrum spreading wireless communication known as frequency hopping is widely used. In frequency-hopping systems for spreading spectrums, when modulated data are up-converted to a radio frequency band (RF band) using an intermediate frequency band (IF band), the frequency of a locally oscillated signal is made to hop continually over a wide frequency range.
FIG. 1 is a block diagram showing a construction of a spread-spectrum communication system of a related-art frequency-hopping scheme shown in Yokoyama “Spread-spectrum communication system” Kagaku Gjutsu Shuppan, May, 1988:564-566. In the following paragraphs, a description will be given of the related-art spread-spectrum communication system with reference to FIG. 1.
A transmitter shown in FIG. 1(a) comprises an encoder 100, a modulator 101, a frequency synthesizer 102, a mixer 103, a band-pass filter 104 and an antenna 105.
A receiver shown in FIG. 1(b) comprises an antenna 106, a band-pass filter 107, a synchronizing circuit 108, a frequency synthesizer 109, a mixer 110, a demodulator 111 and a decoder 112.
The related-art system operates as follows.
Transmission data is input to the encoder 100 for encoding into codewords. The encoder 100 converts a block of transmission data comprising a predetermined number of bits K into a predetermined codeword. The resultant codeword is input to the modulator 101. The modulator 101 subjects the input codeword to data modulation using the MFSK modulation scheme or the DPSK modulation scheme so as to produce a signal of an intermediate frequency (IF). The number of bits k constituting a block of transmission data used to produce a codeword in the encoder 100 and the modulation scheme used in the modulator 101 are selected according to the transmission rate and the quality of communication required.
The IF signal is input to the mixer 103 where the IF signal is mixed by the mixer 103 with an oscillated signal of a predetermined frequency output from the frequency synthesizer 102, so as to produce a signal of a radio frequency (RF) band. The frequency synthesizer 102 is capable of switching between frequencies of the oscillated signal. In frequency-hopping systems, the frequency of the oscillated signal of the frequency synthesizer 102 is changed at a predetermined hopping rate RH (hops/sec) according to a hopping pattern established between a pair of transmitter and a receiver. By changing the frequency of the signal oscillated in the frequency synthesizer 102, the frequency of the RF signal for transmitted data generated in the mixer 103 is also changed at a hopping rate RH (hops/sec) according to the predetermined hopping pattern. As a result, the RF signal is spread over a wider signal band than the bandwidth of the IF signal before up-conversion.
If the hopping rate RH is established as an unreasonably low rate without allowing for a rate at which the codeword is generated by the encoder 100 (hereinafter, referred to as a symbol rate), the frequency remains unchanged for a relatively long period of time, preventing the band-spreading effect for the RF signal provided by frequency hopping from taking effect properly. Accordingly, the hopping rate RH should be set to a suitable level by allowing for the symbol rate. Generally speaking, the higher the rate of data transmission and the symbol rate, the higher the hopping rate RH.
The RF signal subjected to up-conversion and output from the mixer 103 has undesired frequency components removed therefrom by the band-pass filter 104 (hereinafter, referred to as BPF) before being transmitted via the antenna 105.
The receiver shown in FIG. 1(b) receives the RF signal transmitted from the transmitter. The BPF 107 of the receiver removes unnecessary frequency components from the RF signal received via the antenna 106. The synchronizing circuit 108 monitors the variation of the frequency of the RF signal output from the BPF 107 and establishes synchronization between the transmitter and the receiver by detecting a synchronization word included in the RF signal.
When the synchronizing circuit 108 establishes synchronization between the transmitter and the receiver, the frequency synthesizer 109 changes the frequency of the oscillated signal at the predetermined hopping rate RH. The frequency synthesizer 109 of the receiver changes the frequency according to the same hopping pattern referred to by the frequency synthesizer 102 of the transmitter. By synchronizing the hopping pattern of the frequency synthesizer 109 of the receiver with the hopping pattern of the transmitter, the IF signal is recovered from the band-spread RF signal. The RF signal output from the BPF 107 is multiplied for despreading by the mixer 110 by the oscillated signal output from the frequency synthesizer 109, thus producing the IF signal.
The IF signal output from the mixer 110 is subject by the demodulator 111 to a demodulation process. The decoder 112 recovers the original data bits and output the recovered data as received data.
Since the frequency-hopping communication system spreads the signal over a wide band, it has an excellent anti-fading characteristic and provides effective means to improve the quality of communication. Moreover, a frequency-hopping system is superior to other systems in anti-jamming capability and security by employing an arbitrary hopping pattern between a transmitter and a receiver.
In the related-art frequency-hopping spread-spectrum communication system, the RF signal is spread over a wide band by switching between the frequencies of the oscillated signal in the frequency synthesizers 102 and 109 at the predetermined hopping rate RH. Therefore, the hopping rate RH should be set to a suitable level by allowing for the symbol rate of transmission data. When the rate of data transmission is increased and the symbol rate becomes high, the hopping rate RH should be increased. In order for the spectrum-spreading effect provided by frequency hopping to take effect properly in a communication system characterized by high-speed data transmission, a frequency synthesizer capable of switching between the oscillation frequencies at a high speed is required. A technology that provides a frequency synthesizer capable of high-speed switching is disclosed in “Fast Hopping Frequency Synthesizer using Difference of Phase-errors”, The Institute of Electronics Information and Communication Engineers (IEICE) Transactions (Vol.J81-B-II, No.2, February 1998):125-133.
Even when the frequency synthesizer capable of high-speed switching as disclosed in the above reference is used, however, the oscillated signal output from the frequency synthesizer is unstable immediately after the frequency switching. This forces the mixers 103 and 110 to wait for a certain period of time (referred to as frequency switching wait time) in which it is impossible to perform a frequency conversion process. Symbol data of significance to the user cannot be transmitted during this wait time. Therefore, it is necessary to provide a predetermined guard time before and after the frequency switching.
When the hopping frequency RH is increased in the related-art spread-spectrum communication system as described above in accordance with an increase in required data transmission rate, associated reduction in the frequency switching wait time in the frequency synthesizer is limited due to hardware constraints. When the data transmission rate is increased beyond several megabits per second, the frequency switching wait time in the frequency synthesizer grows to a non-negligible level with respect to the interval between frequency switching prescribed by the hopping frequency RH, causing the data transfer rate per carrier to be reduced.
The present invention has been developed with a view to resolving the problem described above and has an objective of providing a transmitter, a receiver and a modulation method for a spread-spectrum communication system with a high frequency hopping rate RH and a high data transmission efficiency, in which the time to wait for the switching of the oscillation signal frequency of the frequency synthesizer is not required so that the frequency switching of the RF signal is effected efficiently.
Another objective is to provide a transmitter, a receiver and a modulation method for a frequency-hopping spread-spectrum communication system in which the bit error rate of data exchanged between the transmitter and the receiver is improved and the quality of communication is favorable.